Added API tests for Docker images.

Added two new API tests for creating repositories that use Docker
content.

**NOTE**: To synchronize a ``Docker`` image, the ``name`` of your custom
  repository **must** match (case matters) the name of the Docker image.
  For example, the
  [WordPress](https://registry.hub.docker.com/_/wordpress/) image would
  require that your repository name be ``wordpress`` as it is displayed
  in its URL.
Merge pull request #1550 from omaciel/docker-api

Added API tests for Docker images.
Added an info blurb linking to new docs
The location of the addition is to keep a greater emphasis on the
most important goal of this document.
Made changes to index for sphinx
Links in this document now work properly in sphinx-based
documentation.